1. Technical Field of the Invention
The present invention relates to a piston ring for use in reciprocating internal combustion engines and in particular relates to piston rings mounted in the ring grooves of aluminum alloy pistons and improved to suppress the aluminum adhesion phenomenon.
2. Description of the Related Art
In recent years, engines must meet increased demands for higher output, high revolutions per minute and longer product service life and must also comply with ever stricter exhaust gas emission regulations. The piston ring must therefore function in a yet harsher operating environment. Many of such piston rings are formed typically with a chromium plating film, nitrided layer or physical vapor deposition film on the upper and lower surfaces.
However, piston rings having a nitrided layer or physical vapor deposition film are prone to wear the ring grooves of the aluminum alloy pistons. In particular, piston rings having nitrided layers are prone to cause aluminum adhesion on the upper and lower surfaces, increasing the wear on the ring grooves.
A method is known however, for suppressing aluminum adhesion in the initial stages of engine operation by coating the upper and lower surfaces of the piston ring with a synthetic resin film containing solid lubricant. However, this synthetic resin film has poor wear resistance, so this method has the drawback that suppression of the aluminum adhesion does not last long.
Methods were also proposed for applying a thin film of artificial diamond material to the top surface and outer circumferential surface and ring groove of the piston and to the piston ring in order to improve durability as described in Japanese Patent Laid-open No. 3-260362. However, no detailed description of this thin film of artificial diamond material is listed in Japanese Patent Laid-open No. 3-260362.
Another method (Japanese Patent Laid-open No. 5-179451) as described next, involved use of a diamond-like carbon film. In this method, in order to restrict the adhesion of ferrite structures, a film having amorphous carbon as the main constituent in which tungsten and/or silicon was dispersed was formed on the sliding surface which mates with a sliding surface formed of iron-type material containing ferrite. This technology is utilized for instance, in the hydraulic valve in power steering mechanisms. However, this Japanese Patent Laid-open No. 5-179451 listed absolutely no information regarding aluminum adhesion on the upper and lower surfaces of the piston rings mating with aluminum alloy pistons.